Protestantism in its strict sense refers to the group of princes and imperial cities who, at the diet of speyer in 1529, signed a protestation against the edict of Worms forbidding the Lutheran teachings in the whole empire. From there, the word Protestant in German speaking areas still refers to Lutheran churches in contrast to Reformed churches, while the common designation for all churches originating from the Reformation is Evangelical.

In a broader sense, Protestantism refers to any of the Christian religious groups, of Western European origin, that broke with the Roman Catholic Church as a result of the influence of Martin Luther, founder of the Lutheran churches, and John Calvin, founder of the Calvinist movement. A third major branch of the Reformation, which encountered conflict with both the Catholics and other Protestants, is sometimes called the Radical Reformation[?], or Anabaptists. Some Western, non-Catholic, Christian groups are labeled as Protestant, even if the sect acknowledges no historical connection to Luther, Calvin, or the Anabaptists.

Protestants generally trace their separation from the Roman Catholic Church to the 1500's, which is sometimes called the magisterial Reformation because it initially proposed numerous radical revisions of the doctrinal standards of the Roman Catholic Church (called the magisterium[?]). The protest erupted suddenly, in many places at once, with distinctive national characteristics in the various regions in which it arose. To some degree, this explosion of protest can be explained by the events of the previous two centuries in Western Europe.

Unrest in the Western Church and Empire, which culminated in the Avignon papacy (1308 - 1378), and then the papal schism[?] (1378-1416),
excited wars between princes, uprisings among the peasants, and widespread concern over corruption in the monastic system. In addition, the humanistic Renaissance was stimulating an unprecedented academic ferment, with a concomittant concern for academic freedom. Earnest theoretical debates were ongoing in the universities concerning the nature of the church, and the proper
source and extent of the authority of the papacy, of councils, and of princes. One of the most disruptive and radical of the new perspectives came first from John Wyclif at Oxford and then from Jan Hus at the University of Prague. Within the Roman Catholic Church, this debate was officially concluded by the Council of Constance (1414-1418), which executed Jan Hus, and posthumously burned Wyclif as a heretic. However, while Constance confirmed and strengthened the Medieval conception of church and empire, it could not entirely resolve the national tensions, nor the theological tensions which had been stirred up during the previous century. Among other things, the council could not prevent schism and the Hussite wars[?] in Bohemia.

To some extent, the protest that began when Luther, an Augustinian monk and professor at the University of Wittenberg, called for reopening of debate on the sale of indulgences[?] (or as tradition holds, literally nailed his 95 theses to the door of the Wittenberg Church), it was a sudden outbreak with new and irresistible force of discontent which had been pushed underground but not resolved.

Parallel to events in Germany, a movement began in Switzerland under the
leadership of Huldreich Zwingli. These two movements quickly agreed on
most issues, as the recently introduced Printing press spread ideas rapidly from place to place but some unsolved differences kept them separate. Some followers of Zwingli believed that the Reformation was too conservative, and moved independently toward more radical positions, some of which survive among modern day Anabaptists. Other Protestant movements grew up along lines of mysticism or humanism (cf. Erasmus), sometimes breaking from Rome or from the Protestants, or forming outside of the churches.

After this first stage of the Reformation, following the excommunication of Luther and condemnation of the Reformation by the Pope, the work and writings of John Calvin were influential in establishing a loose consensus among various groups in Switzerland, Scotland, Hungary, Germany and elsewhere. The separation of the Church of England from Rome under Henry VIII, beginning in 1529 and completed in 1536, brought England alongside the Reformation; but it proceeded there more conservatively than elsewhere in Europe, alternating between traditional and Protestant sympathies for centuries, progressively forging a stable compromise. Thus, the West was permanently divided into Catholic and Protestant.

Jesus Christ (God) is the only referent for adoration and worship. Some believe that Catholics worship saints as well, apparently from the prominent display of statues and images of the saints in Catholic churches and homes, but this belief is in error. Catholics honor saints, and worship God alone.

Against the Catholic view which decided orthodoxy by interpretation of the writings of the Fathers and doctors of the church, through the decisions of councils and the declarations of the bishop of Rome, the Protestants argued that the Bible alone is the Word of God, self-interpreting, and the foundation and test of authority for the Church.

In contrast to the Roman Catholic system of meritorious works, of penance and indulgences, masses for the dead, the treasury of the merits of saints and martyrs, a ministering priesthood, and purgatory, the Protestants argued that every believer is a priest, called to be saints, and obtaining reconciliation with God through faith in Jesus Christ, alone.

Against the Roman Catholic view that faith and works are necessary to preserve the state of grace freely given by God, the Reformers posited that salvation is a gift from God dispensed through Jesus Christ, regardless of merit - for no one deserves salvation.

From the beginning, Protestantism was in agreement against the Roman Catholic doctrine of transubstantiation, which teaches that the substance of the bread and wine used in the sacrificial rite of the Mass, is exchanged for the substance of Christ's body and blood. However, they disagreed with one another concerning the manner in which the believer is united with Christ through the Eucharist. The Lutherans held to a theory called consubstantiation (affirming the substantial presence of Christ in or under the bread). The Reformed according to Zwingli see the Lord's Supper as a memorial ceremony, denying the substantial presence of Christ but affirming that Christ is united to the believer through faith (a view referred to somewhat derisively as memorialism). The Calvinists affirm the real presence of Christ in a manner different from Lutherans, saying that the Church has a new identity from Him in a manner analogous to naming the bread "my body", effecting a spiritual union with the Church, symbolized and given by means of the bread, by the Holy Spirit, through faith, but without changing the bread into Himself.

Protestants can be differentiated according to how they have
been influenced by important movements since the magisterial Reformation and the Puritan Reformation in England. Some of these movements have a common lineage, sometimes directly spawning later movements in the same groups.

The Holiness movement in the 17th and the 18th century, began after the English Puritan Reformation, joined on the continent of Europe the German Pietist movement, and returned to Britain in a changed form through John Wesley and the Methodist Church, as well as through smaller, new groups such as the Quakers. The practice of a spiritual life, often combined with social engagement, predominates in classical Pietism, which was a protest against the Protestant Orthodoxy of the times, which focused on strictly defined doctrines.

Beginning at the end of 18th century, several international revivals of Pietism took place across denominational lines, which are referred to generally as the Evangelical movement. The chief emphases of this movement were individual conversion, personal piety and Bible study, public morality often including Temperance and family values, and Abolitionism, de-emphasis of formalism in worship and in doctrine, a broadened role for laity (including women) in worship, evangelism and teaching, and cooperation in evangelism across denominational lines.

Liberalism is a label for various attempts to accommodate the doctrine and practice, especially of the main branches of the Protestant churches, to the principles of the Enlightenment. These adaptations achieved critical momentum at the end of the 19th century in the Modernist movement and the historical critical Bible exegesis.

In reaction to liberal Bible critique, Fundamentalism arose in the 20th century, primarily in the United States and Canada, among those denominations most affected by Evangelicalism. Fundamentalism placed primary emphasis on the authority and sufficiency of the Bible, and typically advised separation from error, and cultural conservatism, as important aspects of the Christian life.

Neo-evangelicalism is a movement from the middle of the 20th century, that reacted to perceived excesses of Fundamentalism, adding to concern for biblical authority an emphasis on liberal arts, co-operation among churches, Christian Apologetics, and non-denominational evangelization.

Protestants often refer to specific Protestant churches and groups as denominations to imply that they are differently named parts of the whole church. Some denominations, though, are less accepting of others, and some are so unorthodox as to be questioned by most. But there are also denominations where the theological differences are very small. Other denominations are simply regional expressions of the same beliefs found in other places under other names. The actual number of distinct denominations is hard to calculate, but has been estimated to be in the tens of thousands. Various ecumenical movements have attempted cooperation or reorganization of Protestant churches, according to various models of union, but divisions continue to outpace unions. Most denominations claim to have a certain unity with other groups of Christians, but contain doctrines which fundamentally contradict each other.